added test for new function returning true if math-function is a string as well as corresponding functionality

src/main/py/read_math_function.py

@@ -1,4 +1,8 @@
 def read_constant(type, value):
     if type != "c":
         return False
-    return value
+    return value
+
+def is_string(value):
+    result = isinstance(value, str)
+    return result

src/test/py/test_read_math_function.py

@@ -7,6 +7,8 @@ class read_function(unittest.TestCase):
         self.assertEqual(read_constant("c", 42), 42)
     def test_constant_no_c(self):
         self.assertEqual(read_constant("e", "42*e^x"), False)
+    def test_constant_isstring_true(self):
+        self.assertEqual(is_string("42*e^x"), True)
 
 if __name__ == '__main__':
     unittest.main()